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Serverless App Engine:JVM configuration reference

Last Updated:Dec 11, 2025

Learn how to configure JVM stack memory and optimize memory usage to avoid OOM issues.

Configuration reference of stack memory

JVM configuration reference

The following configuration values are for reference only. Adjust the values based on stress tests to optimize system performance.

JVM configuration item	Description	1C 2G	2C 4G	4C 8G	8C 16G
-Xms	The initial size of the heap memory	1G	2560M	4G	10G
-Xmx	The maximum size of the heap memory	1G	2560M	4G	10G
-Xmn	The size of the young generation space	500M	1200M	2G	5G
-Xss	The size of the thread stack space (Default value for JDK 8: 1M)	1M	1M	1M	1M
-XX:MetaspaceSize	The initial size of the metaspace	128M	256M	384M	512M
-XX:MaxMetaspaceSize	The maximum size of the metaspace	128M	256M	384M	512M
-XX:MaxDirectMemorySize	The maximum size of the off-heap memory	256M	256M	1G	1G
-XX:ReservedCodeCacheSize	The size of the code cache	64M	128M	256M	256M

Typical configuration parameters for heap and stack size

Configuration parameter	Description	Example
`-Xmx`	Sets the maximum heap size.	`-Xmx3550m`, sets the maximum available memory for the JVM to 3550 MB.
`-Xms`	Sets the minimum heap size.	`-Xms3550m`, sets the initial memory for the JVM to 3550 MB. We recommend that you set this option to the same value as `-Xmx` to prevent the JVM from reallocating memory after each garbage collection.
`-Xmn`	Sets the size of the young generation.	`-Xmn2g`, sets the size of the young generation to 2 GB. Total JVM heap size = Size of the young generation + Size of the old generation + Size of the permanent generation. In most cases, the permanent generation has a fixed size of 64 MB. Therefore, if you increase the size of the young generation, the size of the old generation is reduced. The value of -Xmn significantly affects system performance. We recommend that you set this option to 3/8 of the total heap size.
`-Xss`	Sets the stack size of the thread.	`-Xss128k`, sets the stack size of each thread to 128 KB. Note If you use JDK 5.0 or later, set this option to 1 MB. If you use earlier JDK versions, set this option to 256 KB. You can adjust the stack size of each thread based on your business requirements. With the same physical memory size, a smaller value for this option allows more threads. The number of threads in a process is limited by your operating system. In most cases, a process can have 3,000 to 5,000 threads.
`-XX:NewRatio=n`	Sets the ratio of the young generation to the old generation.	`-XX:NewRatio=4`, sets the ratio of the young generation (including Eden and two Survivor spaces) to the old generation (excluding the permanent generation). In this case, the young generation occupies 1/5 of the heap.
`-XX:SurvivorRatio=n`	Specifies the ratio of the eden space to the two survivor spaces in the young generation.	`-XX:SurvivorRatio=4`, sets the ratio of the eden space to the survivor spaces in the young generation. In this case, each survivor space occupies 1/6 of the young generation.
`-XX:MaxPermSize=n`	Sets the maximum size of the permanent generation.	`-XX:MaxPermSize=16m`, sets the size of the permanent generation to 16 MB.
`-XX:MaxTenuringThreshold=n`	Sets the age of the objects in the young generation.	`-XX:MaxTenuringThreshold=0`, sets the maximum age of garbage. In this case, the objects in the young generation bypass the survivor spaces and overflow directly into the old generation. This improves the efficiency of applications whose old generation sizes are large. If you set this option to a large value, the objects in the young generation are copied multiple times in the survivor spaces. This increases the period of time during which the objects stay in the young generation. In addition, frequent garbage collection may occur in the young generation.

Configure garbage collectors

Typical configuration parameters for throughput-oriented GC

Configuration parameter	Description	Example
`-XX:+UseParallelGC`	Instructs the JVM to use the parallel GC for the young generation.	`-Xmx3800m -Xms3800m -Xmn2g -Xss128k -XX:+UseParallelGC -XX:ParallelGCThreads=20`, `-XX:+UseParallelGC` This configuration is effective only for the young generation. In this example, the young generation uses concurrent collection, while the old generation still uses serial collection.
`-XX:ParallelGCThreads`	Specifies the number of threads that can be used simultaneously by the parallel GC for garbage collection. Note We recommend configuring this value to equal the number of processors.	`-Xmx3800m -Xms3800m -Xmn2g -Xss128k -XX:+UseParallelGC -XX:ParallelGCThreads=20`, `-XX:ParallelGCThreads=20` indicates that the number of threads for the parallel collector is set to 20.
`-XX:+UseParallelOldGC`	Instructs the JVM to use the parallel GC for the old generation. Note If you use JDK 6.0, you can select the parallel GC for the old generation.	`-Xmx3550m -Xms3550m -Xmn2g -Xss128k -XX:+UseParallelGC -XX:ParallelGCThreads=20 -XX:+UseParallelOldGC`, `-XX:+UseParallelOldGC` indicates that the parallel collection is used for the old generation.
`-XX:MaxGCPauseMillis`	Sets the maximum pause time of each garbage collection for the young generation. The JVM automatically adjusts the size of the young generation to maintain the value of this option.	`-Xmx3550m -Xms3550m -Xmn2g -Xss128k -XX:+UseParallelGC -XX:MaxGCPauseMillis=100`, `-XX:MaxGCPauseMillis=100` sets the maximum pause time of each garbage collection for the young generation to 100 ms.
`-XX:+UseAdaptiveSizePolicy`	Enables the adaptive size policy. If you configure this option, the parallel GC automatically adjusts the size of the young generation and the ratio of the Survivor spaces to achieve the best latency or optimal garbage collection frequency. We recommend that you always enable the adaptive size policy for the parallel GC.	`-Xmx3550m -Xms3550m -Xmn2g -Xss128k -XX:+UseParallelGC -XX:MaxGCPauseMillis=100 -XX:+UseAdaptiveSizePolicy`

Typical configuration parameters for response time-oriented GC

Configuration parameter	Description	Example
`-XX:+UseConcMarkSweepGC`	Instructs the JVM to use the concurrent mark sweep (CMS) GC for the old generation. Note If you configure `-XX:+UseConcMarkSweepGC`, we recommend that you use `-Xmn` to set the size of the young generation.	`-Xmx3550m -Xms3550m -Xmn2g -Xss128k -XX:ParallelGCThreads=20 -XX:+UseConcMarkSweepGC -XX:+UseParNewGC`
`-XX:+UseParNewGC`	Specifies the parallel GC for the young generation. The parallel GC and the CMS GC can be used for the young generation at the same time. If you use JDK 5.0 or later, the JVM automatically configures this option based on your system configurations. In this case, you do not need to configure this option.	`-Xmx3550m -Xms3550m -Xmn2g -Xss128k -XX:ParallelGCThreads=20 -XX:+UseConcMarkSweepGC -XX:+UseParNewGC`
`-XX:CMSFullGCsBeforeCompaction`	The CMS GC does not compact or manage the heap space. Therefore, fragmentation occurs after the CMS GC runs for a specific period of time. As a result, the running efficiency of applications may decrease. This option specifies the number of times that a CMS GC must run through the heap before the heap space is compacted or managed.	`-Xmx3550m -Xms3550m -Xmn2g -Xss128k -XX:+UseConcMarkSweepGC -XX:CMSFullGCsBeforeCompaction=5 -XX:+UseCMSCompactAtFullCollection`, `-XX:CMSFullGCsBeforeCompaction=5`, indicates that the heap space is compacted or managed after the GC runs 5 times.
`-XX:+UseCMSCompactAtFullCollection`	Enables the compaction for the old generation heap. Note If you enable this option, the fragmentation problem can be prevented, but the system performance may be degraded.	`-Xmx3550m -Xms3550m -Xmn2g -Xss128k -XX:+UseConcMarkSweepGC -XX:CMSFullGCsBeforeCompaction=5 -XX:+UseCMSCompactAtFullCollection`

Typical configuration parameters for auxiliary GC

Configuration parameter	Description
`-XX:+PrintGC`	The option to print GC logs.
`-XX:+PrintGCDetails`	The option to print GC details.
`-XX:+PrintGCTimeStamps`	The option to print the GC timestamp from the time when the JVM starts up to the current date. See the following example: `0.855: [GC (Allocation Failure) [PSYoungGen: 33280K->5118K(38400K)] 33280K->5663K(125952K), 0.0067629 secs] [Times: user=0.01 sys=0.01, real=0.00 secs]`
`-XX:+PrintGCDateStamps`	The option to print the GC timestamp as a date. See the following example: `2022-01-27T16:22:20.885+0800: 0.299: [GC pause (G1 Evacuation Pause) (young), 0.0036685 secs]`
`-XX:+PrintHeapAtGC`	The option to print the heap information before and after garbage collection.
`-Xloggc:../logs/gc.log`	The option to specify the output path of the log files.